Conventionally, the number of internal combustion engines equipped with a variable valve device is increasing. Hereinafter, the internal combustion engine is referred to as an engine. The variable valve device is installed in a drive train from a crankshaft to a camshaft that mechanically operates at least one of an intake valve and an exhaust valve. The variable valve device modulates or varies valve movement, such as valve opening timing, valve closing timing, valve timing, and valve lift. In one of the variable valve devices, the variable valve device is powered by fluid, such as oil, supplied by the engine. The variable valve device may be considered as a hydraulic actuator. The variable valve device is installed in the drive train so as to change the camshaft position in a rotational direction or an axial direction. For controlling the variable valve device, a control valve may be disposed in oil lines through which the fluid is supplied or discharged.
A variable valve timing device is sometimes known as a variable valve device. The variable valve timing device is referred to as a VVT. The VVT may be also referred to as a variable cam timing device, a VCT, since it changes a phase difference of the camshaft with respect to the crankshaft of the engine. The VVT varies valve timing for at least one of an intake valve and an exhaust valve in the engine. Valve timing means at least one of opening timing and closing timing of the valve. In a simple configuration of the VVT, since the VVT shifts the phase of the camshaft, both the opening and closing timings are shifted simultaneously. The VVT is also considered as a hydraulic actuator of which fluid supply is controlled by the control valve. The control valve may include a duty driven actuator which adjusts opening and closing conditions of the valve in accordance with a central signal duty cycle value A control apparatus for the VVT calculates the duty cycle value of the control signal so that the control signal adjusts the control valve to make actual valve timing approach a target valve timing. For this purpose, the controller may use a conventional feedback control method, in which the duty cycle value can be obtained based on a difference between target valve timing and actual valve timing. In a preferred arrangement, the VVT has an advancing chamber and a retarding chamber. The oil is supplied to the advancing chamber when the valve timing is advancing and is discharged from the advancing chamber when the valve timing is retarding. The oil is supplied to the retarding chamber when the valve timing is retarding and is discharged from the retarding chamber when the valve timing is advancing. The control valve switches between a supplying mode and a discharging mode, and adjusts an amount of supplying or discharging. The control valve may be provided by a plurality of electromagnetic valves or a multi-port spool valve. The control valve complementarily switches the modes for the advancing chamber and the retarding chamber. In the case of the multi-port spool valve, the modes are switched in response to the duty cycle value of the control signal. The amount of fluid may be referred to by its oil pressure. As a result, the VVT advances or retards the actual valve timing.
JP2001-164964A, U.S. Pat. No. 6,431,131, JP2007-107539A, and U.S. Pat. No. 7,004,128 disclose VVTs. The disclosed VVTs have a dead band on a response characteristic. In one typical response characteristic, a characteristic between the duty cycle value and a changing speed of the valve timing appears as a non-linear characteristic that includes the dead band. In the dead band, the changing speed of the valve timing is almost 0 (zero) or significantly slow compared to outside of the dead band. In other words, in the dead band, the changing speed of valve timing is slow with respect to a changing amount of the duty cycle value compared to outside of the dead band. Therefore, the VVT demonstrates no response or significantly slow response in the dead band.
In order to overcome such disadvantage, JP2001-164964A and U.S. Pat. No. 6,431,131 disclose a sliding mode control method. According to the disclosure, the controller calculates a control amount by using the following steps during deviation between target valve timing and actual valve timing that falls within a range of the dead band. A feedback control amount is calculated by the sliding mode control method. A fundamental control amount, base duty cycle amount, is corrected based on a steady deviation between target valve timing and actual valve timing. Then, the control amount for a control valve is set by summing the fundamental control amount and the feedback control amount.
In order to overcome a disadvantage of the dead band, JP2007-107539A and U.S. Pat. No.7,004,128 disclose several methods. According to the disclosure, the duty cycle, value for a control valve is oscillated. Further, the duty cycle value for the control valve is offset in accordance with a dead band width that is a width of the dead band indicated by a difference of duty cycle values. Alternatively, a control gain is increased within the dead band.